Many attempts in the past have been made to perform beneficiation on coal in order to extract the portion of the coal which will give the cleanest combustion process to keep undesirable emissions (such as sulfur dioxide) to a minimum, and minimize ash slagging and fouling problems. The problem facing the coal preparation engineer is the difficult removal of all minerals and sulfur from coal. Stated another way, it must be accepted that present techniques with which to eliminate the mineral and sulfur constituents of coal also eliminate an economically significant percentage of the consumable part of the coal. Obviously, this economic penalty of coal loss is often unacceptable. If a combustion process can utilize the "dirty" fraction of the total supply concomitant with the consumption of the "clean" fraction, a deep cleaning procedure, or "cream skimming", can be justified. Again, in other words, the problem is to provide a system which will concomitantly utilize both the dirty and clean fractions of the total supply of coal to economic advantage.
Setting aside the availability of anthracitic coal, the principal fuel factor in pulverized-coal-fired boiler design is the characteristics of the coal ash behavior. Ironically, those coals in the U.S., with the higher reactivity, require larger furnaces because the mineral matter in these coals (lower rank) is such that they require lower furnace temperatures to prevent slagging/fouling problems. Obviously, in these boilers, clean coal combustion will enable the reduction of their size and cost. Conversely, fluidized bed combustor design, as compared to pulverized-coal-fired boiler design, is relatively unaffected by the quantity and quality of coal mineral matter. Temperatures in typical fluidized bed combustors are maintained at 1550 F. This is below the initial melting temperature of practically all coal ash, thereby obviating problems due to ash slagging/fouling. Additionally, fluidized beds can be operated with limestone, or dolomite, mixed with the coal in the bed material to provide sulfur capture in the bed. From the standpoint of fuel properties, then, the fluidized bed combustor can much more easily tolerate a dirty fuel without the need for enlarging the combustor, or making other modifications specifically to accommodate the dirty fuel. In summation, what is needed is a system for processing raw coal and supplying it, as fuel, to a pulverized-coal-fired furnace and a fluidized bed furnace for plenary combustion of the coal.